The present application relates to a process for the catalytic epoxidation of olefins with hydrogen peroxide in a continuous flow reaction system.
From EP-A 100 119 it is known that olefins can be converted by hydrogen peroxide into olefin oxides if a titanium-containing zeolite is used as catalyst.
Unreacted hydrogen peroxide cannot be recovered economically from the epoxidation reaction mixture. Furthermore, unreacted hydrogen peroxide involves additional effort and expenditure in the working up of the reaction mixture. The epoxidation of propene is therefore preferably carried out with an excess of propene and up to a high hydrogen peroxide conversion. In order to achieve a high hydrogen peroxide conversion it is advantageous to use a continuous flow reaction system. Such a reaction system may comprise either one or more flow reactors or an arrangement of two or more mixing reactors connected in series. Examples of mixing reactors are stirred tank reactors, recycle reactors, fluidized bed reactors and fixed bed reactors with recycling of the liquid phase.
In order to achieve a high reaction rate a high propene concentration in the liquid phase is necessary. The reaction is therefore preferably carried out under a propene atmosphere at elevated pressure with the effect that a multiphase reaction system is in general present.
Furthermore, the epoxidation of olefins with hydrogen peroxide is like most oxidation reactions highly exothermic. Thus, precautions have to be taken to ensure sufficient removal of the heat generated by the exothermic reaction in order to control the reaction. This problem is especially pronounced in continuous flow systems using fixed bed reactors. Moreover conversion and product selectivity in epoxidation reactions of olefins are highly susceptible to temperature changes with the effect that efficient temperature control is of uppermost importance.
In WO 97/47614 with reference to example 8 reaction of propene with hydrogen peroxide using a fixed bed tubular reactor having a cooling jacket in up-flow operation is described. But yield and product selectivity are still insufficient for commercial purposes.
Especially in highly exothermic reactions like epoxidation reactions, effective removal of the heat of reaction is very important. When using fixed bed tubular reactors with cooling jacket like in WO 97/47614 it might become difficult to control heat generation within the catalyst packing inside the reactor. One possibility to overcome this problem is to use tube bundle reactors wherein the catalyst is either (a) packed within the single tubes or (b) outside the single tubes. To ensure the uniform heat dissipation that is essential in exothermic reactions in the first case (a) tube diameter has to be small and in latter case (b) the distance between single tubes has to be small. Both possibilities create problems when designing the reactor. Operation of those tube bundle reactors having a high number of single tubes is likewise difficult since these reactors are susceptible to blocking and fouling. Furthermore, filling with catalyst to ensure uniform packing of the catalyst bed and exchange of deactivated catalyst for regeneration is becoming increasingly difficult with increased number of tubes or reduced distance between single tubes.
EP-A 659 473 describes an epoxidation process wherein a liquid mixture of hydrogen peroxide, solvent and propene is led over a succession of fixed bed reaction zones connected in series in down-flow operation. No temperature control means are present within the reactor to remove the generated heat from the single reaction zones. Thus, each reaction zone can be considered as an independent adiabatic reactor. In each reaction zone the reaction is performed to a partial conversion, the liquid reaction mixture is removed from each reaction zone, is led over an external heat exchanger to extract the heat of reaction, and the major portion of this liquid phase is then recycled to this reaction zone and a minor portion of the liquid phase is passed to the next zone. At the same time, gaseous propene is fed in together with the liquid feed stock mixture, is guided in a parallel stream to the liquid phase over the fixed bed reaction zones, and is extracted at the end of the reaction system in addition to the liquid reaction mixture as an oxygen-containing waste gas stream. Although this reaction procedure enables the propene oxide yield to be raised compared to conventional tubular reactors without the temperature control described in EP-A 659 473, it nevertheless involves considerable additional costs on account of the complexity of the reaction system required to carry out the process.
From U.S. Pat. No. 5,849,937 a process for epoxidation of propene using hydroperoxides especially organic hydroperoxides is known. The reaction mixture is fed to a cascade of serially connected fixed bed reactors in down-flow regime with respect to each single reactor. Similarly to the teaching of EP-A 659 473 in each reactor only partial conversion is accomplished and the reactors are not equipped with heat exchange means. As in EP-A 659 473, the reaction heat is removed by passing the effluent from each reactor through heat exchangers prior to introducing the reaction mixture to the next fixed bed reactor in series thereby adding to the complexity of the reaction system.
The disadvantages of the reaction systems as discussed in EP-A 659 473 and U.S. Pat. No. 5,849,937 are the complexity and thus the increased costs for investment and the high susceptibility to changes of process parameters like flow velocity due to the adiabaticly operated independent reaction zones and reactors, respectively.
WO 99/29416 discloses a reactor for the catalytic reaction of gaseous reaction media having a plate heat exchanger whereby the catalyst bed is located between the heat exchange plates and the cooling medium or heating medium is passed counter currently to the gaseous reaction phase through the heat exchange plates. There is neither reference to liquid phase or multiphase reaction media in general nor to oxidation or epoxidation reactions.
In view of the cited prior art, an object of the present invention is to provide a process for the epoxidation of olefins that results in improved conversion and product selectivity compared to WO 97/47614 while avoiding the disadvantages of the teachings of EP-A 659 473 and U.S. Pat. No. 5,849,937 which can be candied out using conventional reaction systems.
The above and other objects of the invention can be achieved by a process for the catalytic epoxidation of olefins with hydrogen peroxide in a continuous flow reaction system, wherein the reaction mixture comprising at least one liquid phase is passed through a fixed catalyst bed positioned between parallel heat exchange plates and the reaction heat is at least partially removed during the course of the reaction by passing a cooling medium through the heat exchange plates.
A particularly preferred embodiment of the present invention refers to a process for the catalytic epoxidation of propene with hydrogen peroxide in a continuous flow reaction system conducted in a multiphase reaction mixture comprising a liquid aqueous hydrogen peroxide rich phase containing methanol and a liquid organic propene rich phase, wherein the reaction mixture is passed through a fixed catalyst bed positioned between parallel heat exchange plates in down-flow operation mode and the reaction heat is at least partially removed during the course of the reaction by passing a cooling medium through the heat exchange plates.
The present inventors have surprisingly discovered, that by using a reactor with a bundle of parallel heat exchange plates wherein a fixed bed of catalyst is positioned between the heat exchange plates, the epoxidation of olefins with hydrogen peroxide can be conducted with high olefin oxide selectivity at high hydrogen peroxide conversion compared to tubular reactors with a cooling jacket. Without wishing to be bound by theory it is believed that this surprising effect is attributed to a more uniform heat dissipation when using the reactor type of the present invention.
Compared to tube bundle reactors the dimensions of the plate bundle reactor to be used in the process of the present invention are considerably reduced at the same space-time yield. Thus investment costs are considerably lower. Furthermore the reactor to be used according to the present invention is less susceptible to blocking and fouling compared to tube bundle reactors.
Although WO 99/29416 describes some advantages of a reactor having parallel heat exchange plates it is not derivable from that prior art that especially in liquid phase or multiple phase exothermic reaction systems the selectivity at high conversion can be increased compared to reactor types that are commonly used for example in epoxidation reactions.